This invention relates to aqueous well drilling fluids and more particularly to improved dispersants for aqueous well drilling fluids.
When drilling wells, particularly oil and gas wells it is common practice to circulate a liquid medium into the borehole. The liquid medium, commonly referred to as a drilling fluid or a drilling mud, serves several functions in the well-drilling process. For example, the drilling fluid serves as a vehicle for removing drilling cuttings from the borehole. Another function of the drilling fluid is to cool and lubricate the drill bit. Other functions are to support part of the weight of the drill string and to transmit hydraulic power to the drill bit. Another consideration is that the drilling fluid should be formulated and applied in such a manner that minimal formation damage results and therefore, minimal clogging of the oil-bearing strata occurs. To accomplish these functions the drilling fluid should have three properties: It must provide weight, i.e. hydrostatic head; it must have good flow properties, i.e. good rheological properties; and it must provide filtration control.
A drilling fluid is essentially one of three types; a solids laden fluid with an aqueous continuous phase; a solids laden fluid with a hydrocarbon base; and gaseous. The present invention concerns the first category, that is well-drilling fluids having a continuous aqueous phase, commonly referred to as aqueous drilling fluids.
One of the principal problems encountered in well-drilling operations is that the drilling process requirements continually change due to continuously changing drilling environments. For example, as the depth of the well increases the temperature and pressure in the region of the drill bit increases. Furthermore, contaminants such as drill cuttings, formation water and salts enter the drilling zone. Also, as the drilling proceeds through different strata the mineral constitution of the earth changes. To minimize any adverse effect from these factors additives are incorporated into the drilling fluid from time to time. The present invention relates to one of these additives, a dispersant (also commonly referred to as a "thinning agent").